1. Field of the Invention
The present invention relates to an image display system. More particularly, the present invention relates to an image display apparatus with one or more spatial light modulators that include microstructures shorter than wavelength of an incident light for enhancing the contrast of an image.
2. Description of the Related Arts
Even though there are significant advances of the technologies implementing an electromechanical mirror device as a spatial light modulator (SLM) in recent years, there are still limitations and difficulties when it is employed to provide a high quality image. Specifically, when the images are digitally controlled, the image quality is adversely affected due to the fact that the images are not displayed with sufficient number of gray scales.
An electromechanical mirror device is drawing a considerable interest as a spatial light modulator (SLM). The electromechanical mirror device consists of “a mirror array” arranging a large number of mirror elements. In general, the mirror elements from 60,000 to several millions are arranged on a surface of a substrate in an electromechanical mirror device. Referring to FIG. 1A, an image display system 1 including a screen 2 is disclosed in a reference U.S. Pat. No. 5,214,420. A light source 10 is used for generating light energy for illuminating the screen 2. The generated light 9 is further concentrated and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate light 9 into a column of light 8. A spatial light modulator (SLM) 15 is controlled on the basis of data input by a computer 19 via a bus 18 and selectively redirects the portions of light from a path 7 toward an enlarger lens 5 and onto screen 2. The SLM 15 has a mirror array arranging switchable reflective elements 17, 27, 37, and 47 being consisted of a mirror 33 connected by a hinge 30 on a surface 16 of a substrate in the electromechanical mirror device as shown in FIG. 1B. When the element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along the path 4 to impinge on the screen 2 so as to form an illuminated pixel 3. When the element 17 is in another position, the light is not redirected toward screen 2 and hence the pixel 3 is dark.
Each of mirror elements constituting a mirror device to function as a spatial light modulator (SLM) and each mirror element comprises a mirror and electrodes. A voltage applied to the electrode(s) generates a coulomb force between the mirror and the electrode, thereby making it possible to control and incline the mirror and the mirror is “deflected” according to a common term used in this specification for describing the operational condition of a mirror element.
When a mirror is deflected with a voltage applied to the electrodes to control mirror, the deflected mirror also changes the direction of the reflected light in reflecting an incident light. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. The present specification refers to a state of the mirror when a light of which almost the entirety of an incident light is reflected to a projection path designated for image display as an “ON light”, while referring to a light reflected to a direction other than the designated projection path for image display as an “OFF light”.
And a state of the mirror that reflects a light of an incident light in a manner that the ratio of the light reflected to a projection path (i.e., the ON light) and that reflected so as to shift from the projection path (i.e., the OFF light) is a specific ratio, that is, the light reflected to the projection path with a smaller quantity of light than the state of the ON light is referred to as an “intermediate light”.
According to a convention of present specification, it defines an angle of rotation along a clockwise (CW) direction as a positive (+) angle and that of counterclockwise (CCW) direction as negative (−) angle. A deflection angle is defined as zero degree (“0°”) when the mirror is in the initial state, as a reference of mirror deflection angle.
Most of the conventional image display devices such as the devices disclosed in U.S. Pat. No. 5,214,420 implement a dual-state mirror control that controls the mirrors at a state of either ON or OFF. The quality of an image display is limited due to the limited number of gray scales. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the least pulse width as control related to the ON or OFF state. Since the mirror is controlled to operate in an either ON or OFF state, the conventional image display apparatuses have no way to provide a pulse width to control the mirror that is shorter than the control duration allowable according to the LSB. The least quantity of light, which determines on the basis of the gray scale, is the light reflected during the time duration according to the least pulse width. The limited gray scale leads to a degradation of the image.
Specifically, FIG. 1C shows an exemplary control circuit for controlling a mirror element according to the disclosures made in U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors; while transistors M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is based of a Static Random Access Switch Memory (SRAM) design. The transistor M9 connected to a Row-line receives a DATA signal via a Bit-line. The memory cell 32 written data is accessed when the transistor M9 that has received the ROW signal on a Word-line is turned on. The latch 32a consists of two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states, that is, a state 1 is Node A high and Node B low, and a state 2 is Node A low and Node B high.
The mirror is driven by a voltage applied to the electrode abutting a landing electrode and is held at a predetermined deflection angle on the landing electrode. An elastic “landing chip” is formed at a portion on the landing electrode, which makes the landing electrode contact with mirror, and assists the operation for deflecting the mirror toward the opposite direction when a deflection of the mirror is switched. The landing chip is designed as having the same potential with the landing electrode, so that a shorting is prevented when the landing electrode is in contact with the mirror.
Each mirror formed on a device substrate has a square or rectangular shape and each side has a length of 4 to 15 um. In this configuration, a reflected light that is not controlled for purposefully applied for image display is however inadvertently generated by reflections through the gap between adjacent mirrors. The contrast of image display generated by adjacent mirrors is degraded due to the reflections generated not by the mirrors but by the gaps between the mirrors. As a result, a quality of the image display is worsened. In order to overcome such problems, the mirrors are arranged on a semiconductor wafer substrate with a layout to minimize the gaps between the mirrors. One mirror device is generally designed to include an appropriate number of mirror elements wherein each mirror element is manufactured as a deflectable micromirror on the substrate for displaying a pixel of an image. The appropriate number of elements for displaying image is in compliance with the display resolution standard according to a VESA Standard defined by Video Electronics Standards Association. Alternately, the number in compliance with the television broadcast standards. In the case in which the mirror device has a plurality of mirror elements corresponding to WXGA (resolution: 1280 by 768) defined by VESA, the pitch between the mirrors of the mirror device is 10 um and the diagonal length of the mirror array is about 0.6 inches.
The control circuit as illustrated in FIG. 1C controls the micromirrors to switch between two states and the control circuit drives the mirror to oscillate to either an ON or OFF deflected angle (or position) as shown in FIG. 1A.
The minimum quantity of light controllable to reflect from each mirror element for image display, i.e., the resolution of gray scale of image display for a digitally controlled image display apparatus, is determined by the least length of time that the mirror controllable to hold at the ON position. The length of time that each mirror is controlled to hold at an ON position is in turn controlled by multiple bit words. FIG. 1D shows the “binary time periods” in the case of controlling SLM by four-bit words. As shown in FIG. 1D, the time periods have relative values of 1, 2, 4, and 8 that in turn determine the relative quantity of light of each of the four bits, where the “1” is least significant bit (LSB) and the “8” is the most significant bit. According to the PWM control mechanism, the minimum quantity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” for holding the mirror at an ON position during a shortest controllable length of time.
In a simple example with n bits word for controlling the gray scale, one frame time is divided into (2n−1) equal time slices. If one frame time is 16.7 msec, each time slice is 16.7/(2n−1) msec.
Having set these time lengths for each pixel in each frame of the image, the quantity of light in a pixel which is quantified as 0 time slices is black (no the quantity of light), 1 time slice is the quantity of light represented by the LSB, and 15 time slices (in the case of n=4) is the quantity of light represented by the maximum brightness. Based on quantity of light being quantified, the time of mirror holding at the ON position during one frame period is determined by each pixel. Thus, each pixel with a quantified value which is more than 0 time slices is displayed by the mirror holding at an ON position with the number of time slices corresponding to its quantity of light during one frame period. The viewer's eye integrates brightness of each pixel so that the image is displayed as if the image were generated with analog levels of light.
For controlling deflectable mirror devices, the PWM calls for the data to be formatted into “bit-planes”, where each bit-plane corresponds to a bit weight of the quantity of light. Thus, when the brightness of each pixel is represented by an n-bit value, each frame of data has the n-bit-planes. Then, each bit-plane has a 0 or 1 value for each mirror element. In the PWM described in the preceding paragraphs, each bit-plane is independently loaded and the mirror elements are controlled according to bit-plane values corresponding to them during one frame. For example, the bit-plane representing the LSB of each pixel is displayed as 1 time slice.
When adjacent image pixels are displayed with a very coarse gray scales caused by great differences of quantity of light, thus, artifacts are shown between these adjacent image pixels. That leads to the degradations of image qualities. The degradations of image qualities are specially pronounced in bright areas of image when there are “bigger gaps” of gray scale, i.e. quantity of light, between adjacent image pixels. The artifacts are caused by a technical limitation that the digitally controlled image does not obtain sufficient number of the gray scale, i.e. the levels of the quantity of light.
The mirrors are controlled either at ON or OFF position. Then, the quantity of light of a displayed image is determined by the length of time each mirror holds, which is at the ON position. In order to increase the number of the levels of the quantity of light, the switching speed of the ON and OFF positions for the mirror must be increased. Therefore the digital control signals need be increased into a higher number of bits. However, when the switching speed of the mirror deflection is increased, a stronger hinge for supporting the mirror is necessary to sustain a required number of switches of the ON and OFF positions for the mirror deflection. Furthermore, in order to drive the mirrors provided strengthened hinge toward the ON or OFF positions, applying a higher voltage to the electrode is required. The higher voltage may exceed twenty volts and may even be as high as thirty volts. The mirrors produced by applying the CMOS technologies probably is not appropriate for operating the mirror at such a high range of voltages, and therefore the DMOS mirror devices may be required. In order to achieve a control of higher number of the gray scale, a more complicated production process and larger device areas are required to produce the DMOS mirror. Conventional mirror controls are therefore faced with a technical problem that the good accuracy of gray scales and range of the operable voltage have to be sacrificed for the benefits of a smaller image display apparatus.
There are many patents related to the control of quantity of light. These patents include U.S. Pat. Nos. 5,589,852, 6,232,963, 6,592,227, 6,648,476, and 6,819,064. There are further patents and patent applications related to different sorts of light sources. These patents include U.S. Pat. Nos. 5,442,414, 6,036,318 and Application 20030147052. Also, The U.S. Pat. No. 6,746,123 has disclosed particular polarized light sources for preventing the loss of light. However, these patents or patent applications do not provide an effective solution to attain a sufficient number of the gray scale in the digitally controlled image display system.
Furthermore, there are many patents related to a spatial light modulation that includes the U.S. Pat. Nos. 2,025,143, 2,682,010, 2,681,423, 4,087,810, 4,292,732, 4,405,209, 4,454,541, 4,592,628, 4,767,192, 4,842,396, 4,907,862, 5,214,420, 5,287,096, 5,506,597, and 5,489,952. However, these inventions do not provide a direct solution for a person skilled in the art to overcome the above-discussed limitations and difficulties.
In view of the above problems, an invention has disclosed a method for controlling the deflection angle of the mirror to express higher gray scales of an image in a US Patent Application 20050190429. In this disclosure, the quantity of light obtained during the oscillation period of the mirror is about 25% to 37% of the quantity of light obtained during the mirror is held on the ON position at all times.
According to such control, it is not particularly necessary to drive the mirror at high speed. Also, it is possible to provide a higher number of the gray scale using a low elastic constant of the hinge that supports the mirror. Hence, such control makes it possible to reduce the voltage applied to the landing electrode.
An image display apparatus using the mirror device described above is broadly categorized into two types, i.e. a single-plate image display apparatus equipped with only one spatial light modulator and a multi-plate image display apparatus equipped with a plurality of spatial light modulators. In the single-plate image display apparatus, a color image is displayed by changing in turn the color, i.e. frequency or wavelength of projected light is changed by time. In a multi-plate the image display apparatus, a color image displayed by allowing the spatial light modulators corresponding to beams of light having different colors, i.e. frequencies or wavelengths of the light, to modulate the beams of light; and combined with the modulated beams of light at all times.
In the single-plate image display apparatus and multi-plate image display apparatus, a configuration is such that the light illuminates a wider zone than the array of spatial light modulators (SLMs). As a result, it is possible to display a bright and uniform image across an entire image. Such a configuration, however, allows the reflection light from parts illuminated by other than the arrayed light modulator elements (e.g., mirrors) to incident to the projection lens. As a result, the contrast of the image is degraded.
Also, the light arriving at a substrate from a gap between the adjacent light modulation elements is reflected on the substrate to become an unnecessary reflection light. And the unnecessary reflection light entering the projection path decreases the contrast of an image.
An improvement of the image is an important problem of an image display apparatus, for which various contrivances are devised for spatial light modulators (SLMs) to solve the problem.
There is a method for layering a light absorption mask on a part other than the array of light modulator elements of a spatial light modulator (SLM) or forming a light absorption layer on light modulation elements. Such a configuration prevents an unnecessary reflection light from entering the projection path.
As an example, in a mirror device arraying a plurality of deflectable mirror elements as light modulation elements, formed is an anti-reflection layer or light absorption layer on the surface other than the reflection surface of the mirror, such as the back thereof and the top surface of the substrate retaining the mirror. Meanwhile, in a spatial light modulator (SLM) employing a liquid crystal as light modulation element, formed is an anti-reflection layer or light absorption layer in the components not contributing to an image generation, such as a transistor and the wall surface between the liquid crystal elements.
In order to make the anti-reflection layer as described above function effectively to an incident light possessing a wide wavelength band, however, a plurality of layers with different thick nesses must be formed. This consequently is faced with the problem of increasing the number of producing processes. On the other hand, a coating of a black material that is the simplest method for forming a light absorption layer is difficult to apply to a micro electro mechanical system (MEMS) device possessing a very minute structure. As an example, there is a problem associated with the process for depositing a thin layer of carbon black in a specific place.
Note that there are following disclosure related to the problem described above.    1. B. S. Thornton, “Limit of the moth's eye principle and other impedance-matching corrugations for solar-absorber design”, JOURNAL OF THE OPTICAL SOCIETY OF AMERICA VOLUME65, NUMBER3, MARCH 1975, 267-270.    2. S. J. WILSON and M. C. HUTLEY, “The optical properties of ‘moth eye’ antireflection surfaces”, OPTICA ACTA, 1982, Vol. 29, No. 7, 993-1009    3. Eric B. Grann, M. G. Moharam, and A. Pommet, “Optimal design for antireflective tapered two-dimensional sub wavelength grating structures”, OPTICAL SOCIETY OF AMERICA Vol. 12, No. 2, February 1995, 333-339    4. Philippe Lalanne and G Michael Morris, “Antireflection behavior of silicon subwavelength periodic structures for visible light”, Nanotechnology, 8, 1997, 53-56    5. Yuzo Ono, Yasuo Kimura, Yoshinori Ohta, and Nobuo Nishida, “Antireflection effect in ultrahigh spatial-frequency holographic relief gratings”, APPLIED OPTICS, Vol. 26, No. 6, 15 Mar. 1987, 1142-1146 [Japan Patent Application] 2003-294910A, Sanyo Electric Co., Ltd. [Japan Patent Application] 2001-27505, Japan Science and Technology Agency